1. Field of the Invention
The present invention relates to a semiconductor light emitting element and a semiconductor light emitting device.
2. Related Background Art
Semiconductor light emitting elements are used in display boards, back-lighting of liquid crystals, etc. as light sources of low power consumption, high efficiency and high reliability to substitute for lamps. Especially, InGaAlP compound semiconductor materials are direct transition type materials and capable of emitting bright light in the region from red to green. Therefore, InGaAlP compound semiconductor light emitting elements are actually used in red stop lamps of cars, red and yellow lamps of traffic lights, and so on, as relatively luminous elements. In such a semiconductor light emitting element, emission is attained by recombination of holes and electrons when a current is injected in the active layer inside the light emitting layer. Part of the light generated in the active layer is taken out externally from the surface of the semiconductor element. The semiconductor light emitting element is usually mounted on a lead frame including a reflection board, and sealed with a mold of epoxy resin having a lens-like configuration. Emission property is controlled by reflecting light from the semiconductor light emitting element with the reflection board and converging the reflected light with a lens.
Although InGaAlP compound semiconductor materials ensure high-luminance emission, they are formed on opaque GaAs substrates. Therefore, if GaAs substrates are maintained, absorption of light by GaAs substrates will occur. To cope with this problem, a method for obtaining a relatively high-luminance semiconductor light emitting element has been developed recently, which first forms InGaAlP compound semiconductor materials on an opaque GaAs substrate, thereafter bonds a transparent GaP substrate and instead removing the opaque GaAs substrate. The term “transparent” is used here to mean transparency to light emitted from the light emitting element.
Even by bonding a transparent substrate, this method could not successfully make the best use of the excellent properties of InGaAlP compound semiconductor materials, and luminance of the element is still insufficient. Its reason lies in that the light extracting efficiency is low. More specifically, light generated from an InGaAlP active layer inside a semiconductor element is radiated in all directions over 360°. However because of a difference in refractive index between the semiconductor crystal and epoxy resin sealing it, part of the light is reflected by the semiconductor interface and cannot extracted externally of the semiconductor crystal. Assuming that the refractive index of the semiconductor crystal is 3.3 and that of the epoxy is 1.5, the critical angle is θc=sin−1(1.5/3.3)=27° according to the Snell's law, and rays of light that impinge the interface between the semiconductor and the resin at angles larger than 27° undergo total reflection, and it is impossible to extract this light from inside the semiconductor crystal. Usually, semiconductor light emitting elements are cubes (hexahedrons), and if light can be extracted ideally from all surfaces, approximately 28% of light should be extracted. Actually, however, extractable quantity of light is less than 28% because: two kinds of electrodes of the n-type and the p-type are formed on surfaces of the semiconductor crystal; the n-type or p-type electrode is bonded to the reflection board with an adhesive; and part of light impinging the electrode surfaces is absorbed by the electrode alloy layer.
As a structure overcoming this problem and enhancing the light extraction efficiency, a method using a transparent substrate and appropriately shaping the transparent electrode is currently under researches. This method is disclosed in, for example, Japanese Patent Laid-Open Publication No. hei 10-341035. FIG. 16 shows this method. On a first surface 301 of a transparent p-type GaP substrate 300, as illustrated, a light emitting layer 314 is formed. The light emitting layer 314 has a structure including a p-type semiconductor layer, active layer and n-type semiconductor layer sequentially stacked on the first surface 301 of the p-type GaP substrate 300. The light emitting layer 314 is supplied with a current from the p-side electrode 309 and the n-side electrode 310. In response to injection of the current, the active layer of the light emitting layer 314 emits light, and this light is extracted from the second surface 302 of the substrate 300. In order to increase the total quantity of extracted light, the semiconductor light emitting element shown in FIG. 16 is so shaped that orientation of side surfaces 303 of the transparent substrate 300 are offset from the direction vertical to the light emitting layer 314. In this configuration of the element, the shaped side surfaces 303 of the transparent substrate 300 reflects the light emitted from the light emitting layer 314 toward the second surface 302. Therefore, this method certainly enables extraction of more light from the second surface 302. Additionally, from the edge surface 303, it is possible to extract part of light reflected by the second surface 302, and thereby increases the quantity of entire extracted light. Furthermore, since the shaped side surfaces 303 ensure extraction of light generated inside without suffering multiple reflections by the crystal interface, absorption by the active layer of the light emitting layer 314 or absorption by ohmic contact portions decreases. As such, the embodiment shown in FIG. 16 enhances the light extraction efficiency by using the second surface 302 of the substrate 300 as light extracting surface and adequately shaping the substrate 300.
However, the semiconductor light emitting element of FIG. 16 and a semiconductor light emitting device using it involve some problems such as insufficient production yield and short lifetime irrespectively of their high efficiency of extraction of light.
More specifically, since the element of FIG. 16 uses the second surface 302 as the light extracting surface, the pn junction in the light emitting layer 314 is close to the lower mount surface 304 as illustrated. Therefore, when the mount surface 304 of the element is mounted on a lead frame with an electrically conductive mount agent (adhesive), edge surfaces 305 of the on junction in the light emitting layer 314 are short-circuited, and this causes the problem of decreasing the production yield. If the mount agent is reduced not to creep on the edge surfaces 305 of the light emitting layer 314 to eliminate that problem, then the bonding force decreases, and this invites another problem that the mount surface 304 of the light emitting element easily separates from the lead frame during long-time electrical excitation and it reduced in lifetime.
Additionally, the element of FIG. 16 needs dicing for adequately shaping the substrate 300. In the dicing process, however, the dicing blade may produce damage layers (crystal defects) on the side surfaces 303 and inside the substrate 300. Thus, during long-time electrical excitation, crystal defects grow from the damage layers, the light emitting layer 314 becomes liable to break, and the lifetime is shortened. Additionally, if the semiconductor light emitting element including such damage layers is sealed with epoxy resin, crystal defects grow from the shaped side surfaces 303 of the substrate 300 having such damage layers to the light emitting layer 314 due to the resin stress during electrical excitation, and may invite deterioration of the optical output. Here again, the lifetime will be shortened. Taking this problem into account, encapsulation, which seals the light emitting element with a soft gel resin such as silicone, is employed. However, since the outside of the encapsulation is sealed by a hard resin such as epoxy, interfacial separation, which is separation of silicone and epoxy along their interface, will occur, and the optical output will decrease during electrical excitation. Here again the lifetime will be shortened. Therefore, it has been believed that the lifetime is inevitably shortened in case the substrate 300 is machined.
As such, it has generally been believed that the semiconductor light emitting element including a substrate adequately shaped to enhance the light extraction efficiency is inevitably subject to a decrease of the production yield and a reduction of the lifetime, although the light extraction efficiency is certainly enhanced.